Learning Perl Objects, References amp;amp; Modules [Electronic resources] نسخه متنی

9.14 Restricting a Method to Class-Only or Instance-Only

Setting the name of an unnameable
generic Horse is probably not a good idea; neither
is calling named on an instance. Nothing in the
Perl subroutine definition says "this is a class
method" or "this is an instance
method." Fortunately, the ref
operator lets you throw an exception when called incorrectly. As an
example of instance- or class-only methods, consider the following:

use Carp qw(croak);
sub instance_only {
ref(my $self = shift) or croak "instance variable needed";
... use $self as the instance ...
}
sub class_only {
ref(my $class = shift) and croak "class name needed";
... use $class as the class ...
}

Here, the ref function returns true for an
instance or false for a class. If the undesired value is returned,
you'll croak, which has the added
advantage of placing the blame on the caller, not on you. The caller
will get an error message like this, giving the line number in their
code where the wrong method was called:

instance variable needed at their_code line 1234

While this seems like a good thing to do all the time, practically no
CPAN or core modules add this extra checking. Maybe
it's only for the ultra-paranoid.